BOSTON (AP) — They’re nestled amid bustling downtowns and tucked into nondescript strip malls across quiet suburbs. Brothels posing as massage parlors and Asian spas have been part of the American landscape for decades, hidden in plain sight.

But the Florida prostitution sting that ensnared New England Patriots owner Robert Kraft last week is a reminder of the human trafficking and abuse taking place behind the darkened windows of many of these storefronts — and how challenging they are to address.

The case also highlights how police and prosecutors are increasingly using a broad range of approaches, including deeper investigations into wider criminal networks, crackdowns on online sites where johns trade detailed sex reviews and enforcement of stricter civil codes on the massage industry, anti-trafficking activists said.

“You’re fighting against a multibillion-dollar industry that’s very, very good at being strategic and keeping their business going,” said Stephanie Clark, executive director at Amirah, a nonprofit that runs a safe house for women escaping sex trafficking in Massachusetts, where illegal massage parlors have proliferated. “They are always 10 steps ahead.”

As many as 9,000 illegal massage parlors currently operate in more than 1,000 cities nationwide, fueling a roughly $3 billion industry, according to the Polaris Project , a nonprofit that runs the National Human Trafficking Hotline.

Most of the prostitutes are women from China and South Korea in their mid-30s to late 50s who have entered the country illegally, are deeply in debt and are drawn into sex work through a combination of lies, threats and other forms of coercion, the organization said.

The massage parlor in Jupiter, Florida where Kraft, a 77-year-old Massachusetts billionaire, was videotaped engaging in sex acts is typical of the model.

Tucked into a pedestrian strip mall in an affluent oceanside community, the Orchids of Asia Day Spa employed mostly Chinese immigrant women and was linked to at least nine other storefronts from Palm Beach to Orlando.

Authorities say the women averaged about 1,500 clients a year, were given no days off and were not allowed to leave the site, where many also lived. Palm Beach State Attorney Dave Aronberg described it as “modern-day slavery.”

“We need to get beyond the whack-a-mole strategy of taking out one retail location at a time,” said Bradley Myles, Polaris’ CEO. “We need to see multi-state investigations that take a longer look, follow the money and build these organized crime cases.”

Law enforcement officials in California, which is home to roughly a third of the nation’s illegal massage parlors, as well as jurisdictions in Minnesota, Utah and Washington are also landing similar large cases, Myles said.

In Massachusetts, about half of the more than 50 people charged under the state’s 8-year-old anti-human trafficking law were involved in illegal massage businesses or residential brothels, according to state Attorney General Maura Healey’s office.

In one recent case, a 38-year-old woman was charged with running a lucrative human trafficking and money laundering operation across six Asian massage parlors in the suburbs north of Boston.

Prosecutors said Xiu J. Chen recruited Asian women from New York and arranged their appointments, transportation and housing, where they typically slept on mattresses on the floor. Chen was sentenced to five years in prison in December.

But in New York, another hub of the illegal massage parlor industry, major busts involving sex traffickers remain frustratingly elusive, despite police rolling out a new human trafficking strategy in 2017 promising to crack down on customers and traffickers rather than sex workers, said Chris Muller of Restore NYC, a nonprofit that works with immigrant sex trafficking survivors.

A silver lining is that authorities are helping connect more women with groups like Restore NYC that can help get them on a path to citizenship and break the grip of traffickers, who oftentimes hold their passports and immigration documents as collateral, he said.

New York police said they investigated 79 illegal massage parlors for nuisance violations in 2018, but didn’t say how many of those storefronts were ultimately shutdown. Police data also shows prostitution arrests declined more than 60 percent from 2016 numbers while arrests of their customers rose nearly 180 percent.

New York is also among the places seeing growing support for decriminalizing and even legalizing sex work , as is the case in parts of Nevada and Europe. But anti-trafficking groups and local officials appear focused, for now, on more attainable legislative goals.

Delaware and North Carolina, for example, recently classified massage parlors as health businesses, making them subject to regular inspections and other sanitation and safety requirements. Lawmakers in Illinois, New Jersey, Texas and a dozen other states are also weighing stricter regulations on the massage industry this year.

In Massachusetts, Healey backs proposed legislation to close a loophole that authorities say has allowed illegal spas to operate as unregulated “bodyworks” operations, despite passage of statewide massage parlor requirements in recent years.

At the city and county level, codes limiting operating hours for massage parlors or banning features like buzzer-controlled front doors and back-door entrances have been used in recent years to shutter hundreds of storefronts in San Francisco, San Jose and other parts of California. But officials acknowledge these local measures often just push the industry into neighboring communities without those requirements.

Federal and state prosecutors, meanwhile, have gone after the johns who post Yelp-style reviews about their massage parlor experiences on online message boards.

In the Seattle-area, for example, authorities shut down a local site called The Review Board and charged dozens of people, including reviewers and massage parlor operators, on prostitution-related offenses in 2016.

The Department of Justice said federal sex trafficking legislation enacted last year empowers states to go after problematic sites. It also highlighted recent cases in which federal prosecutors shut down prostitution-related websites and brought charges against their owners, including last year’s takedown of the notorious escort listing website Backpage.com.

For former massage parlor sex worker Jasmine Grace Marino, the solution is simple: End the demand for paid sex.

The 38-year-old New Hampshire resident says she was pressured to work at sites in Connecticut and Maine in her 20s by her then-boyfriend, who eventually became her pimp. She walked away after five years, wrote a book about her experience and also runs Bags of Hope, a Boston-based ministry that helps women who have been trafficked or are dealing with addiction or homelessness.

“Men need to have these conversations,” Marino said. “Look at Robert Kraft. Even being billionaire and winning all those championships, he’s still not satisfied and has to fill that need illegally. Something is broken in there for these men.”